Field of the Invention
The present invention relates to, inter alia, a relative rotational angular displacement detection device used to detect a relative rotational angular displacement of a pair of rotatable members arranged coaxially with each other.
More specifically, the present invention relates to a relative rotational angular displacement detection device preferably for use in a power assist system for, e.g., a power assist wheelchair, a power assist bicycle, a power steering wheel, etc. The present invention also relates to a torque detection device including the relative rotational angular displacement detection device, and a torque control device including the relative rotational angular displacement detection device. It also relates to a power assist wheelchair, a power assist straddle-type vehicle, and a power steering device equipped with the torque control device.
Description of the Related Art
For example, in a conventional manual wheelchair, a pair of hand rims are arranged outside of a pair of right and left rear wheels, respectively, and coaxially connected thereto. When a user rotates the hand rim, the rotational force is transmitted to the wheel to move the wheelchair. In recent years, for the purpose of reducing the burden of moving the hand rim by a user, a power assist system has been developed, in which the most appropriate assisting force corresponding to the manual force for moving the hand rim is transmitted to a driving wheel by an electric motor.
According to this system, the manual force for moving the hand rim of the wheelchair and the rotational force of the electric motor outputted in accordance with the manual force are combined to rotate the wheels, which enables easy moving of the wheelchair. This kind of power assist system can be applied not only to a wheelchair but also to a power assist bicycle, a power steering device of an automobile, etc.
This kind of power assist system is provided with a detection device for sensing a torque by detecting a relative rotational angular displacement of a pair of rotatable members coaxially arranged with each other in a relatively rotatable manner. As a device for detecting such a relative rotational angular displacement or a relative rotational torque, Japanese Unexamined Laid-open Patent Application Publication No. 2008-249366 discloses the following device. The device includes a pair of first and second shaft members arranged coaxially with each other, a cylindrical magnet fixed to the first shaft member, a pair of yoke rings fixed to the second shaft member, a pair of magnetic flux collector rings each arranged so as to surround each yoke ring and each having a magnetic flux collecting projection, and a magnetic sensor arranged between the magnetic flux collecting projections and configured to detect magnetic flux changes occurring in the yoke rings in accordance with the relative angular displacements of the first and second shaft members.
In the relative rotational angular displacement detection device, the first shaft member is coaxially provided with the cylindrical magnet so as to rotate together with the first shaft member. The cylindrical magnet includes magnetic poles, i.e., N-poles and S-poles, magnetized in a radial direction of an axis of rotation and arranged alternately in a circumferential direction of the axis of rotation. The second shaft member is provided with the pair of yoke rings which rotate together with the second shaft member. Each yoke ring includes triangular shaped magnetic pole claws corresponding to the N-poles and S-poles.
Each magnetic pole claw is arranged outside of the cylindrical magnet so as to face the pole of the cylindrical magnet in the radial direction of the axis of rotation. The pair of yoke rings are arranged such that the magnetic pole claws of one of the yoke rings and the magnetic pole claws of the other of the yoke rings are arranged so as to oppose in an axial direction of the axis of rotation and arranged alternately in the circumferential direction. A pair of magnetic flux collector rings each for collecting the magnetic fluxes generated in each yoke ring are arranged radially outside of the corresponding yoke rings.
When the first shaft member and the second shaft member are relatively rotated, the position of each yoke ring relative to the magnetic pole of the cylindrical magnet is changed. This causes magnetic flux changes between the magnetic flux collector rings. The magnetic flux changes are detected by the magnetic sensor.